1. Field of the Invention
The present invention relates to an image adjustment processing in an image forming apparatus, and, more particularly to improvement of accuracy of image adjustment processing.
2. Description of the Related Art
In recent years, color MFPs are becoming increasingly popular year by year. In particular, since office documents are prepared in color, an increasing number of color copies and prints are used. In the case of the color MFPs, it is more important to always keep colors and densities constant. However, in general, a density characteristic of a printer engine tends to change as time elapses. Thus, some of the color MFPs have a function for gamma automatic adjustment (calibration) to keep colors and gradations as constant as possible.
Specifically, the gamma automatic adjustment is a function of outputting, in general, a pattern of a regular gradation, inputting the pattern with a scanner, calculating an inverse function or the like from a read value, and setting a gamma correction curve for adjusting an output image quality. In general, this gradation pattern for gamma correction is provided by using a gradation pattern in which the density changes from bright to dark, and this gradation pattern is formed such that gradation patches placed at fixed intervals (at fixed density changing amounts) are arranged in the density changing direction.
When, for example, a charging characteristic or the like of a developer is deteriorated and durable lives of consumables are nearly over (life end), a gradient in an intermediate density region of a gamma curve of the printer engine becomes steep and gradation properties in a highlight region (bright region) and a dark region (dark region) tend to be lost.
Even if a correction pattern is outputted in a state in which the gradation properties in the highlight region and the dark region of the printer engine fall in this way, since it is difficult to reproduce gradations in the highlight region and the dark region, a correction curve cannot be calculated accurately. Consequently, it is impossible to perform highly accurate image adjustment.
For example, when an 8-bit signal is equally divided into N to output a pattern as shown in FIG. 10, at the beginning of a printer life, since reproduction of gradations in the highlight region and the dark region can be performed, no problem occurs. However, near a life end, since densities are substantially the same in several gradations from the beginning on the highlight side and in last several gradations on the dark side, it is impossible to calculate an accurate correction value. Actually, a correction value is calculated by performing interpolation or the like. As a result, since a gradation value that cannot actually be reproduced is selected, it is impossible to calculate an accurate correction curve and perform highly accurate image adjustment processing.